Which iPad should you buy? Well, first of all, realize that everything I say here is going to be colored by my experience. I’m going to try to give you advice, but there are plenty of factors that I might not see.
It seems to me that there are mainly three disadvantages to the iPad mini versus iPad with Retina display (4th generation). (Two versus the iPad 2.)
Although the iPad mini display has the same number of pixels as the iPad 2, they are smaller. So text is going to be smaller. In lots of cases you can adjust the text size. Even so, if you have trouble reading smaller print, I think you might be happier with the one of the larger iPads.
The extra pixels of the iPad with Retina display (4th generation) means that it is practical to read some PDF files on it that are less practical for anyone to read on an iPad 2 or iPad mini. This is important to me because I carry, reference, and read a lot of PDFs (mainly role-playing game material) on my iPad. For most people, though, I don’t know if the Retina display—as beautiful as it is—makes much difference.
Likewise, tap targets on the iPad mini will be smaller. They’re still big enough in general. If, e.g., you find yourself often having trouble tapping targets accurately on an iPhone, you might be happier with one of the larger iPads.
The third disadvantage to the iPad mini is performance (i.e. CPU/GPU speed). The iPad mini and the iPad 2 are pretty much the same in this department, but the iPad with Retina display (4th generation) is much better. It’s hard to generalize about whether this will really matter to you, but certain types of games are probably one of the biggest reasons you might want the extra performance. Music creation and recording apps might be another.
The big advantage of the iPad mini is weight. The smaller size, for most people, isn’t going to make it that much more portable than the larger iPads. For something that you normally hold in your hands while using, though, being lighter can mean a lot.
In general, the iPad mini is superior to the iPad 2. Unless the text size or tap-target size mentioned above matter to you, you should probably get an iPad mini rather than an iPad 2.
Generally, I would not suggest the 16GB models unless you’re going to be sticking mainly to Mail and Safari. (Which is another knock against the iPad 2, which is only offered in 16GB configurations.) If you’re going to be carrying music and movies on it, you’ll probably want more space. If you’re going to be playing games on it, you’ll probably want more space.
Also, there’s no way to upgrade an iPad’s storage. So, if in doubt, it isn’t a bad idea to err on the side of more.
There’s an argument to be made everyone should spend the $130 for a cellular radio. If you decide not to use it, you’re out $130. If you don’t have it and decide to want it, though, you have to buy a whole new iPad. Personally, I’ve been happy with the Wi-Fi models, and I use my iPhone’s Personal Hotspot on the occasions when I want my iPad to be online where there’s no Wi-Fi. People with less access to Wi-Fi, however, might find it very useful.
Me? I’m sticking with my 3rd generation iPad for now. The big, Retina display, high performance, and maximum storage capacity are important to me. So, if I do decide to upgrade soon, I’d go with the iPad with Retina display (4th generation).